The present invention relates generally to amplifier circuits, and in particular to techniques for controlling current in an amplifier of a column driver.
The liquid crystal display has become ubiquitous and well known, driven in part by popular applications such as laptop personal computers, car navigational displays, and flat panel displays for personal computers. In each of these applications, a column driver circuit enables the operation of each liquid crystal display unit. Liquid crystal displays comprise a plurality of individual picture elements, called pixels, which are uniquely addressable in a row and column arrangement. The column driver circuitry provides driving voltage to the columns of the liquid crystal display. In a typical application, a 13.3 inch XGA liquid crystal display comprises 1024 3-color columns, for a total of 3072 individual columns. In a representative arrangement, these columns are driven by eight 384-column driver chips.
The physics underlying liquid crystal display technology calls for an alternating polarity in the driving voltage. That is to say, if a column of the display is driven at +5 volts for a specific period of time, then this same column is driven at xe2x88x925 volts during the subsequent time interval. In such an arrangement, the peak to peak voltage is 10 volts, but the sum of the individual driving voltages for any given cycle is 0 volts. Failure to drive a liquid crystal display in such fashion can cause the display to degrade until it becomes no longer usable.
Column driver circuitry components act as intermediaries between the digital format of the electronics that process information and the analog format of the display presenting the results to the user. Accordingly, the column driver circuitry includes a digital to analog converter component that converts the digital signals of the processing unit, bus and memory into an analog signal. However, this analog signal must be capable of driving the liquid crystal display. While some arrangements drive the liquid crystal display columns directly from the digital to analog converter, another technique is to use an amplifier interposed between the converter and the display in order to provide improved driving characteristics for the display.
While certain advantages to conventional approaches are perceived, opportunities for further improvement exist. For example, in many conventional approaches to amplifier design, errors introduced into the output signal of the amplifier are not uniform throughout a substantially large portion of the operation of the circuit. Error characteristics can vary among the different regions of operation of the amplifier. In flat panel display applications, where the amplifier is used to buffer a digital analog converter input that drives columns or rows of the flat panel display, these errors can adversely affect image quality because the view can perceive changes in the image when the voltage having the error is switched.
What is needed are improved techniques for controlling current flow in an amplifier circuit.
The present invention provides improved techniques for controlling current flow in an amplifier circuit. Specific embodiments provide steering of analog outputs of digital to analog converters in order to drive columns of an LCD display. Embodiments can provide a full range of voltage output to drive an LCD display without necessitating a full range amplifier configuration. Further, many specific embodiments can be realized in smaller space on an IC chip than in conventional technologies.
In a representative specific embodiment, the present invention provides a current steering circuit. The current steering circuit comprises a current input node coupled to a first circuit path, the first circuit path drawing current from the current input node during a first mode of operation. A comparator is coupled to the current input node. The comparator draws negligible amount of current from the current input node during the first mode of operation. However, the comparator draws a significant amount of current from the current input node during a second mode of operation so as to divert current from the first circuit path. A current mirror is coupled to the comparator. The current mirror maintains a current flow through a second circuit path during the second mode of operation but not during the first mode of operation.
In some specific embodiments, the current steering circuit further comprises a reference node coupled to the comparator. The reference node provides a reference voltage. A voltage input node can be coupled to the comparator as well. The voltage input node provides an input voltage. The comparator places the current steering circuit in the first mode of operation or the second mode of operation when the input voltage is at a predetermined level relative to the reference voltage.
Specific embodiments of the current steering circuit further comprise a p-type differential amplifier coupled to the first circuit path, and an n-type differential amplifier coupled to the second circuit path. The current steering circuit may further comprise a current source coupled to the current input node in specific embodiments.
The reference voltage can be established by a diode-connected transistor, for example. The voltage input node can include an input terminal of a p-type differential amplifier in specific embodiments.
In a further representative specific embodiment, the present invention provides a current steering circuit. The current steering circuit comprises a current input node coupled to a first circuit path. The first circuit path draws current from the current input node during a first mode of operation. A first transistor is coupled to the current input node. The first transistor draws negligible current from the current input node during the first mode of operation. The first transistor draws current from the current input node during a second mode of operation to divert current from the first circuit path. A second transistor coupled to receive current drawn by the first transistor from the current input node during the second mode of operation is also part of the current steering circuit. The second transistor forms a current mirror with a third transistor that is coupled to a second circuit path. The current steering circuit also includes a reference node providing a reference voltage to a fourth transistor, and a fifth transistor forming a current mirror with the fourth transistor. The fifth transistor supplies a reference current to the first transistor. The amount of the reference current can be related to the reference voltage. The current steering circuit is placed in the first mode of operation or the second mode of operation depending on a voltage level on a voltage input node relative to the reference voltage.
In specific embodiments, the current steering circuit is placed in the first mode of operation when the voltage level on the voltage input node is lower than the reference voltage, and in the second mode of operation when the voltage level on the voltage input node is higher than the reference voltage. In some specific embodiments, the current steering circuit is placed in the first mode of operation when the voltage level on the voltage input node is higher than the reference voltage, and in the second mode of operation when the voltage level on the voltage input node is lower than the reference voltage. In specific embodiments, the first, second, third, fourth, and fifth transistors are MOS transistors.
In a yet further representative specific embodiment, the present invention provides a method for maintaining a substantially constant error in an output voltage sourced by an amplifier that comprises a first circuit and a second circuit. Depending on embodiment, the first and second circuits can be a p-channel amplifier and an n-channel amplifier, respectively, or an n-channel amplifier and a p-channel amplifier, respectively. The method comprises driving an output voltage in a first region of operation in each of the first circuit and the second circuit. Sensing a condition wherein an input voltage, Vin, reaches a reference voltage, Vref, is also part of the method. Further, switching over from a first region of operation to a second region of operation in each of the first circuit and the second circuit is part of the method. The reference voltage, Vref, can be made sufficiently large to provide a substantially constant error within the output voltage.
Numerous benefits are achieved by way of the present invention over conventional techniques. Embodiments can provide a substantially constant error term over a substantially greater region of operation in a voltage output to drive an LCD display. Specific embodiments can provide rail-to-rail voltage range of operation. Embodiments can be configured to switch between regions of operation at input voltages substantially close to the rail. In specific embodiments, a non-constant error region is limited to a portion of the region of operation of a display in which the human eye is not especially sensitive.
These and other benefits are described throughout the present specification. A further understanding of the nature and advantages of the invention herein may be realized by reference to the remaining portions of the specification and the attached drawings.